1. Field of the Invention
The present invention relates to a copolymer for optical compensation film. More particularly, the present invention relates to a copolymer for optical compensation film showing a negative orientation birefringence, having excellent transparency, heat resistance, film strength, and optical properties, and capable of providing a beautiful film suitable for an optical compensation film.
2. Description of the Related Art
As an optical compensation film of a liquid crystal display, stretched film obtained by uniaxial stretching or biaxial stretching of a resin film is widely used. As a typical example of the optical compensation film, retardation film can be mentioned. A λ/2 plate to convert the vibration direction of the polarized light or a λ/4 plate to convert a circularly polarized light into a linearly polarized light or to convert a linearly polarized light into a circularly polarized light are widely used.
The retardation film is required to show optical compensation in a wide range of vision. It is an extremely important property for the retardation film to have hardly any phase difference even for the incoming light from an oblique direction. Regarding such property required, Patent Literature 1 discloses a liquid crystal display comprising a laminated body of a transparent stretched film having a negative orientation birefringence and a transparent stretched film having a positive orientation birefringence.
Patent Literature 2 discloses a method for widening the view angle of the liquid crystal display by using an optical compensation film structured by laminating a stretched film showing a negative orientation birefringence and a stretched film showing a positive orientation birefringence in a manner so that the slow axes of each of the stretched films are parallel, the optical compensation film having an in-plane phase difference (Re) in the range of 60 to 300 nm, and an orientation parameter (Nz) in the range of 0.5±0.1. In addition, Patent Literature 2 discloses that a stretched film showing a negative intrinsic birefringence is a resin composition of a copolymer of α-olefin and N-phenylmaleimide and an acrylonitrile-styrene copolymer.
As the thermoplastic resin showing a positive orientation birefringence, polycarbonate and an amorphous cyclic polyolefin can be mentioned. They are suitably used for the optical compensation film due to the excellent heat resistance, transparency, film strength, and phase difference development. On the other hand, as the thermoplastic resin showing a negative orientation birefringence, examples of practical application is extremely rare since whichever one of heat resistance, transparency, film strength, and phase difference development is inferior. The ones that have practical applications are mainly structured by bonding a plurality of stretched film showing a positive orientation birefringence with an appropriate angle. Therefore, designation of optical compensation is complicated and its cost is high, while the optical compensation performance is insufficient. From the viewpoint of improvement in optical compensation performance, simplification of optical design, and reduction in cost, a thermoplastic resin showing a negative orientation birefringence which can be practically applied for the optical compensation film is desired.
Regarding such requirements, Patent Literature 3 suggests a thermoplastic resin copolymer having excellent transparency, heat resistance, film formability, film strength, and phase difference development; and a stretched film showing a negative orientation birefringence. It is true that the film formability is superior regarding a general film forming processing, however, films for optical compensation require extremely beautiful film free of foreign substances. When manufactured by melt extrusion, a polymer filter with extremely small sieve is often used for removing foreign substances. Here, the thermoplastic resin copolymer of Patent Literature 3 has high melt viscosity, and thus it tends to remain in the polymer filter. In addition, high temperature is required to avoid pressure loss, which can result in thermal decomposition of the resin causing foams and die lines. Therefore, the range of application was limited.